This invention relates to an apparatus for driving a wire-dot print head in a wire-dot impact printer.
A wire-dot print head comprises a plurality of print wires, and a means for driving the print wires forward so that their ends impact on a sheet of paper. An inked ribbon is interposed between the ends of the print wires and the paper so that the impact of each wire causes the printing of a dot. Characters and graphic designs are printed as a matrix of dots by driving the print wires at appropriate times as the head travels across the paper.
In the well-known spring-release type of wire-dot print head, the means for driving each print wire comprises an armature, a plate spring, and an electromagnet. The plate spring is secured at one end. The print wire is attached to the armature, which is mounted on the free end of the plate spring.
Normally a permanent magnet holds the spring in a flexed position in which the print wire is retracted. When an electric current flows through the electromagnet for a print wire, it produces a magnetic field opposing the field generated by the permanent magnet, thereby releasing the spring. The print wire is thereby driven forward to print a dot.
When the current is removed from the electromagnet, the permanent magnet again attracts the armature, causing the print wire to return to its retracted position in preparation for printing the next dot.
There is a delay between the application of a voltage to the electromagnet (energization of the electromagnet) and the current flowing through the electromagnet because of the inductance in the electromagnet, and there is a delay between the current in the electromagnet and the movement of the print wire because of the inertia of the armature, and the like. If the time of energization is too short, the impact will be weak or absent, causing faint or skipped dots. If the energization time is too long, however, the print wire will be late in returning to its retracted position, then it will be necessary to lengthen the printing cycle. Otherwise the print wires will not be ready for the operation in the next printing cycle.
The optimum energization time depends on a plurality of factors, one of which is the voltage Vcc applied to the electromagnet. A prior-art scheme for controlling the energization time employs a resistor and capacitor connected in series between the power supply terminal Vcc and the ground, with the energization time regulated according to the charging time of the capacitor. This scheme automatically compensates for variations in the power supply voltage Vcc.
This prior-art timing scheme, however, fails to compensate for variations in characteristics of the electromagnets, and magnetic interference inside the print head. As a result, the energization time is not optimum, and the printing quality is not satisfactory. Moreover, to allow for such variations, it is necessary to add a margin to the energization time. Accordingly, on the average the electromagnet is energized for longer than the optimum time. As a result, the prior-art wire-dot print head driving apparatus is unnecessarily slow, consumes unnecessary current, and generates unnecessary heat.